The gases to be treated, resulting from the oxycombustion of fossil fuels, contain compounds of the NO, NO2, SO2, SO3, H2O, CO, CO2, O2, Ar and N2 type.
The presence in this type of gas of traces of so-called acidic gases of NO2 or SO2 type in the gas phase does not pose particular problems with regard to the corrosion of materials. Indeed, those skilled in the art are able to make among ordinary steels a suitable choice of the quality to be accepted according to the operating conditions. In the presence of water vapor, there are neither problems of corrosion, above the dew point. Carbon steel, for example, can be accepted if the conditions are favorable.
On the contrary, the treatment of fluids containing acidic liquids, of nitric acid and/or sulfuric acid type, in particular at temperatures above room temperature, presents corrosion problems.
FIG. 1 shows that the weight loss rates of steel by corrosion in the presence of acidic solutions are high, and that they strongly increase with temperature.
In this case, it is known to use special “ultra-pure” steels with very strict chemical composition specifications, in particular with respect to compounds such as carbon or phosphorus. The disadvantage of these materials is that they are relatively unavailable and are much more expensive than ordinary steels. Steel able to resist such conditions is, for example, 304 NAG, whose price is from three to five times greater than that of ordinary steel.
Hereinafter, the term “ordinary steel” will be used in contrast to these special steels. Among ordinary steels, for example, mention may be made of carbon steel and type 304 or 316L stainless steels.
Another solution consists in coating the adsorber's inner wall with enamel.
This operation is costly and the coating thus obtained is fragile (poor impact strength).
Nevertheless, one or the other solution is used when hot acidic liquids are continuously present.
The drying of gases of the type, for example, resulting from oxycombustion as cited above, in a TSA (temperature swing adsorption) type adsorption method, corresponds to a specific new type of corrosion risk.
Although only gases containing traces of impurities (NOx, SOx) in vapor form are treated, hot acidic liquids can be formed in the adsorber but, and this is what is particular, only in a localized manner and for limited periods of time.
Indeed, the simultaneous adsorption of water and acidic gases leads to the formation of acidic compounds, for example by the following reactions, notably in the presence of oxygen:H2O+SO2+½O2→H2SO4 H2O+SO2→H2SO3 H2SO3+½O2→H2SO4 H2O+2NO2→HNO3+HNO2 2HNO2→H2O+NO+NO2 NO+½O2→NO2 SO2+½O2SO3 
and other reactions including the change of sulfur and nitrogen to higher degrees of oxidation, or to dismutations. These reactions take place preferably in the adsorbent because of the concentration encountered, and lead to the formation of an acidic aqueous phase.
The acidic phase can be stabilized by interaction with the adsorbent, for example by hydrogen bonds, which leads to its gas-phase formation reaction being accelerated and shifted.
It is also of note that the adsorbents can catalyze certain redox reactions, which means that relatively nonadsorbable gaseous substances can also be sensitive to the presence of adsorbents. The catalytic effects depend on the adsorbent and its surface properties: Si—OH groups, Al—OH groups, cations, organic functional groups (acids, alcohols, ketones, lactones, aldehydes), surface defects. Different adsorbents can thus have different properties: activated carbon, zeolites, silica gels, activated or calcined alumina. For a single type of adsorbent, there are also significant variations resulting from its preparation, its chemical composition and its porous structure.
It must be noted that the strong acids thus generated, and especially sulfuric acid, are relatively non-volatile and are hygroscopic. Thus, these acids will accumulate during cycles because the regeneration conditions specific to the drying methods using adsorption are not sufficient to desorb them entirely.
Due to their low volatility, traces of elements can lead to the accumulation of acids that do not immediately come to mind. For example, traces of phosphorus compounds can lead to the formation of stable and relatively nonvolatile phosphoric acid (H3PO4). The list of these acidic compounds can be established from the boiling points found in the tables.
The acids formed may possibly be neutralized by bases present in the gas, and lead to corrosive salts, such as chlorides or phosphates.
Regeneration of the adsorbent by a hot, generally dry, gas utilizes counterflow desorption of the adsorbed water, which then will pass over the zone impregnated with the strong acids accumulated previously. Under these conditions, the hot regeneration gas loaded with moisture will condense part of the water it contains on these hygroscopic acidic compounds.
This will lead to an acidic solution that will fill the porosity of the adsorbent, until an excess aqueous phase is formed. This aqueous and highly acidic phase will be in contact with the adsorbers' walls, indeed will even end up by streaming. Then, the heat front passing, these liquids will essentially vaporize and/or will be found at room temperature.
The hot regeneration gas exiting the bed of adsorbent will be found periodically saturated with water and acidic vapors which will condense on the bottom of the colder adsorber. Here as well, hot acidic liquids will be present cyclically.
Consequently, a problem that arises is to limit the corrosion of adsorbers during TSA drying of a feed gas stream including at least water, SOx and NOx, without having to use the expensive solutions that prevail in the continuous presence of acids in liquid form and at high temperature.